1. Field of the Invention
The present invention relates to a storage battery system consisting of a battery module in which a plurality of storage batteries each using negative electrodes made of, as a main component, a hydrogen storage alloy that electrochemically absorbs and desorbs hydrogen, are arranged in rows, or which comprises a battery group composed of a plurality of such battery modules combined with one another and arranged in rows, and in particular to the heat radiation structure of the storage battery system.
2. Prior Art of the Invention
A nickel-hydrogen storage battery having a negative electrode which is a hydrogen storage electrode made of hydrogen storage alloy or the hydride thereof, and a positive electrode made of nickel oxide, is interchangeable with a nickel-cadmium storage battery but can have a capacity greater than that of the nickel cadmium battery. However, such a nickel-hydrogen storage battery expels a large volume of oxygen gas from its positive electrodes during charging or in particular upon overcharging.
This oxygen gas is reacted with hydrogen occluded in the negative electrode so as be reduced into water. At this time, the temperature of the battery rises due to the exothermic reaction thereof. Further, it has been known that the temperature rise at this time is higher than that of the nickel-cadmium storage battery during charging.
Further, a conventionally used small size cylindrical type nickel-hydrogen storage battery radiates its generated heat, outside of the battery during charging with a high degree of efficiency since the sheath casing is made of metal, and accordingly, the temperature rise of the battery is not high even upon overcharging.
However, a nickel-hydrogen storage battery having an intermediate capacity of about 20 to 300 Ah generates a large volume of heat during charging. The generation of heat is caused by a joule heat due to a resistance, heat of reaction due to electrode reaction and heat of reaction by hydrogen.
At this time, if the efficiency of heat radiation of the battery, outside of the battery, is not high, the generated heat is accumulated in the battery, resulting in abrupt increase in the temperature of the battery.
Further, since the charging and discharging efficiency of the battery is lowered if the temperature of the battery becomes high, and accordingly, the availability of active materials decreases. Thus, it has been found that a desired battery characteristic is sometimes unable to be obtained if the heat generated during charging is not efficiently radiated outside of the battery.
Further, in a storage battery module in which a plurality of storage batteries are stacked one upon another, or in a storage battery system composed of a battery group in which a plurality of such storage battery modules are arranged, the temperatures of the batteries differ from each other depending upon the dispositions of the batteries during charging.
Gas or electrolyte possibly leaks from a battery at a high temperature through a safety valve incorporated in the battery case since the internal pressure of the battery increases, and accordingly, the use life of the entire storage battery system is deteriorated due to the battery from which electrolyte has is been depleted.
Further, a battery module having a relatively small capacity has less charge and discharge current, and accordingly, the temperature rise of the battery is not high. However, the larger the capacity thereof, the larger the charge and discharge current, and further, since the number of batteries constituting the battery module becomes larger, the efficiency of heat radiation of the battery is lowered, thereby the temperature rise of the battery becomes higher.
The structure of the battery system in which the efficiency of heat radiation of a storage battery system having a large capacity and a large number of batteries to be used can be enhanced so as to restrain the temperature rise of the batteries is important.
In order to solve the above-mentioned problems, Japanese Laid-Open Patent No. 3-291867 proposes a storage battery system in which several batteries inevitably generating heat are arranged in rows, and in which spaces through which air flows are defined between the batteries, having the ratio between the width of the spaces between the batteries, and the width of a single battery set in a range of 0.1 to 1.0.
Further, a device for forcibly blowing air into the spaces between the batteries is attached to one side surface of the storage battery system in order to restrain the temperature rise of the batteries during charging.
However, in this arrangement, if the ratio between the width of the spaces between the batteries, and the width of the single battery is increased to 1, although it is preferable in view of the heat radiation, the bulk of the storage battery system increases since the width of the spaces becomes larger, and accordingly there has been raised a problem of lowering the energy density of the battery system (which corresponds to the ratio between the output energy and the bulk of the battery system). Further, since the width of the spaces between the batteries is regulated in accordance with the width of the batteries, that is, the width of the battery casings, the width of the spaces between the batteries varies even though the same electrode groups are used, because the width of the battery casings vary as the thickness of the walls of the battery casings varies. The temperature rise of the batteries during charging depends upon the size of the electrode group stored in the battery casing.
That is, the larger the thickness of the electrode group, the harder it is to radiate the generated heat, that is, the larger the heat is accumulated in the electrode group. Accordingly, the space between the batteries, which influences the efficiency of heat radiation of the storage battery system, should be determined not by the width of the batteries but by the thickness of the electrode group. Further, it is appropriate to regulate the width of spaces between the batteries in accordance with the thickness of the electrode group under such a condition that the thickness-wise sectional area of the electrode group falls in a certain specified range.